1. Technical Field
This invention relates to a system and method for laser welding the ends of thin-walled metal tubes and, optionally, for filling and sealing a contained mass inside the metal tubes by use of a robot and a multidirectional laser. More particularly, the invention relates to a system and method for manufacturing radioactive brachytherapy xe2x80x9cseedsxe2x80x9d that are implantable for use in medical therapy such as the irradiation of tumors.
2. Prior Art
The use of radiation therapy in medicine, particularly in cancer treatment, is well established. Conventional external radiation sources are used to irradiate both cancer tumor bodies and residual cancerous tissues after surgical excision of a solid tumor. Irradiation utilizing exterior radiation sources is limited in usefulness due to the high doses of radiation required and the resultant harmful consequences to surrounding and intervening healthy tissues.
Interstitial brachytherapy (short-range therapy) is also a well-established medical procedure in which a source of radioactivity, often called a xe2x80x9cseed,xe2x80x9d is implanted within or near a tumor or other tissue. This procedure produces more focused radiation dosages and reduces the total radiation dosage required to achieve a desired therapeutic effect, thereby reducing harm to surrounding normal tissues. The seed is generally a hollow, sealed container made of a material that is relatively transparent to radiation. The radiation source within the seed varies with the required medical specifications, the type of radioisotope and the manufacturing process.
Examples of brachytherapy seeds are disclosed in U.S. Pat. No. 4,891,165 to Suthanthrian, U.S. Pat. No. 4,323,055 to Kubiatowicz, U.S. Pat. No. 4,702,228 to Russell, U.S. Pat. No. 5,405,309 to Carden, and U.S. Pat. No. 5,997,463 to Cutrer.
Suthanthrian discloses a brachytherapy seed comprising two hollow cylinders, each having one open end. One cylinder is slightly smaller than the other, allowing the open end of the first to slide into and toward the closed end of the second cylinder. The cylinders desirably fit together tightly.
Kubiatowicz discloses a seed construct wherein the radioactive material is loaded manually and the ends of the titanium seed are welded shut such as by plasma-arc welding.
Russell and Carden disclose the same seed construct, which improves on the Kubiatowicz and Suthanthrian seeds by using form-fitting caps at the ends of a hollow cylindrical titanium seed. These caps are laser welded onto the cylinder body. This configuration produces seed ends that are the same thickness as the walls. While this diminishes the anisotropy of the emitted radiation relative to the Kubiatowicz and Suthanthrian seeds, the cap-cylinder overlap creates a zone of thicker seed material that causes some anisotropy in the emitted radiation.
Cutrer discloses a brachytherapy source and method of manufacture in which a single laser and a rotating chuck are used, either alone or in combination with crimping, to seal radioactive spheres inside metal tubes. The laser is preferably activated in four series of short pulses to complete the welds because welding with a single laser can produce an unstable weld pool that can interfere with sealing and with the radiation pattern exhibited by the resultant product. Cutrer also discloses the provision of an inert gas source to minimize or eliminate oxidation during welding and to cool the weld afterwards.
The prior art construction of brachytherapy seeds also involves manual addition of the radiation source and sealing the seed in a separate, discontinuous step. This manufacturing scheme is labor intensive, time consuming and results in significant radiation exposure to workers. Additionally, the Food and Drug Administration and the Nuclear Regulatory Commission require the radioactive materials to be safely contained so as to prevent leakage and radiation poisoning.
A safe, automated method and apparatus that will facilitate the manufacture of brachytherapy seeds while minimizing worker radiation exposure, improving product quality and increasing manufacturing capacity are therefore needed.
This invention disclosed herein is an automated system and method useful for producing welded end closures in thin-walled metal tubes, and for encapsulating or containing a mass within a small-diameter metal tube. As used herein, the term xe2x80x9cthin-walledxe2x80x9d generally refers to tubes having wall thicknesses less than about 0.5 mm. The encapsulated mass can comprise a solid, liquid or gel, and, as more particularly described below, can comprise a radioactive component to facilitate use in biomedical applications. According to a particularly preferred embodiment of the invention, the small-diameter, thin-walled, metal tubes used in the invention are made of titanium, the mass includes a plurality of radiolabeled spherical metal bodies, and encapsulation or containment is achieved by laser welding the bottom and top ends of the metal tube using an multi-directional laser source to produce brachytherapy seeds.
The system of the invention preferably comprises a plurality of discrete modules or subassemblies serviced by a SCARA-form robot mounted on a stationary base, all housed inside a protective enclosure such as a sealed, negative pressure isolation glove box, which maintains a filtered production environment and provides containment and shielding for any radioactive component of the product. The discrete modules of the system contained inside the enclosure preferably include the SCARA robot, an automated gripper, a tube dispenser, a multi-directional laser welder, a loader-filler, at least one inspection module, and an output receiver. xe2x80x9cSCARAxe2x80x9d is commonly used to refer to a robot having a xe2x80x9cSelectively Compliant Articulated Robot Arm.xe2x80x9d SCARA robots have a cylindrical work envelope, and the articulated support arm permits access to work locations disposed at virtually all locations within the envelope.
The subject system is desirably automated and operated by programmable electronic controllers that cause the various modules to perform functions as disclosed herein, sometimes receiving input from, and sometimes directing signals to, a plurality of sensors and transducers. One primary control computer preferably receives feedback from the robot and various other sensing devices to control the process. A second computer desirably provides information regarding fine visual positioning at the laser welder to the primary control computer.
According to a particularly preferred embodiment of the invention, the laser welder comprises three laser outputs spaced at even intervals around a target location and positioned so that beams routed from a common source through a splitter and fiber optic cables converge in a target zone at substantially the same time to weld the selected end of a metal tube while the tube is held in position by the automated gripper of the robot.
According to a preferred embodiment of the method of the invention, precut metal tubes of desired length, most preferably made of titanium, are retrieved by the automated gripper of the robot from the tube dispenser, moved to the laser welder and welded at the bottom, moved to the loader-filler and loaded with a predetermined mass to be contained inside the tube, inspected to insure proper filling, returned to the laser welder for welding the top of the tube, inspected to insure weld integrity, and then moved to the output receiver and released.
According to a particularly preferred application of the invention, the subject system and method are used to produce brachytherapy seeds for localized radiation treatment of cancer cells. When producing brachytherapy seeds, the contained mass preferably comprises a plurality of spheres coated or impregnated with a radionuclide, or a gel or gelable material containing a radionuclide.